This post explains how to find the OS version which was first installed on a CentOS/RHEL system.

CentOS/RHEL 6

Run grep -m1 “label =” /var/log/anaconda.storage.log to find the original installation OS version.

Current version:

# cat /etc/redhat-release
CentOS Linux release 6.10

Original installation version:

# grep -m1 "label =" /var/log/anaconda.storage.log
  label = EL6.8_x86_64_Disc_1_20160518  size = 0  targetSize = 0

The above shows original installation was CentOS 6.8.

CentOS/RHEL 7 and above

For CentOS/RHEL 7

Run grep -m1 inst.stage2=hd:LABEL /var/log/anaconda/syslog to find the original installation OS version.

Current Version:

# cat /etc/redhat-release
CentOS Linux release 7.9

Original installation version:

# grep -m1 inst.stage2=hd:LABEL /var/log/anaconda/syslog
08:12:14,153 INFO kernel:[    0.000000] Command line: initrd=initrd.img inst.stage2=hd:LABEL=EL-7.2x20Server.x86_64 quiet BOOT_IMAGE=vmlinuz

The above shows original installation was CentOS 7.2.

For CentOS/RHEL 8

Current Version:

# cat /etc/redhat-release
CentOS Linux release 8.3

Original installation version:

# grep -m1 inst.stage2=hd:LABEL /var/log/anaconda/syslog
09:07:12,619 INFO kernel:Command line: BOOT_IMAGE=vmlinuz initrd=initrd.img inst.stage2=hd:LABEL=EL-8-0-0-BaseOS-x86_64 quiet

The above shows original installation was CentOS 8.0.